7.2.2 Optimizing Analyte Separations with a Temperature Program
The goal of chromatography is to separate a complex mixture of compounds. Some separations are relatively simple while others require experimentation to optimize the instrumental settings. Analyte separations are controlled by the temperature settings of the column and oven. Usually the initial temperature of the oven is set at approximately 10 to 15 degrees below the boiling point of the solvent. After injection, the oven temperature may or may not be held at this value for a few minutes. Next, the oven and column temperature is increased as slow as needed to allow separation of the compounds but as fast as possible to minimize the instrument run time. Finally, after all of the analytes have reached the detector, the instrument is usually held at a high temperature to allow any high boiling compounds to exit the column. The key to an adequate separation is to determine each of these temperatures, noting the need to achieve adequate separation in a minimum amount of time, especially in an industrial setting where cost (and time) efficiency is mandatory.
In this experiment, we will show the optimization of the temperature program for a set of hydrocarbons normally found in gasoline, the subject of the next lab.
Experimental Procedures:
Chemicals and Supplies:
-A Pasteur pipet for each analyte
-One 10-mL volumetric flask
-Neat (pure) samples of benzene, decane, ethyl benzene, n-heptane, isooctane, toluene, m-xylene, and o-xylene.
Instrumental Settings:
-GC-FID Settings (Flame Ionization Detector)
-Capillary Column: DB-5: Poly(phenylmethyldimethyl) siloxane (5 % phenyl)
30 m x 0.53 mm; 1.5 mm phase coating
-Injection Volume: 1.00 mL
-Splitless Injection for: 1.00 min.
-Split Flow Rate: 50 mL/min.
-Column Flow: 1.2 mL/min.
-Linear Velocity: 14 cm/s
-Injector Temperature: 230C
-Detector Temperature: 250C
-Oven Program: varied as described below.
Sample Preparation:
The dilution solvent will be pentane because it has a very low boiling point and most other dilution solvents would co-elute with one or more analytes. Prepare a qualitative standard, as described below, for injection into the GC.
-Add two drops of each analyte to approximately 10 mL of pentane, cap in an air-tight vial, and mix the solution.
-Inject this solution into the GC using a variety of temperature programs. Start with a relatively low temperature isothermal program (50 C) for an extended time (20-30 minutes). Next, use a relatively high temperature isothermal program (150 C for 15 minutes). You will not obtain complete separation for either of these programs. Finally, use a temperature program starting from a temperature just below the boiling point of your analyte with the lowest boiling point and program an increase of 5 C per minute to a final temperature approximately 10 C above the boiling point of your analyte with the highest boiling point. Refer to the next experiment in section 7.2.3 for optimum temperature programming instructions. When all peaks have been separated, the elution order will be: benzene, n-heptane, isooctane, toluene, ethyl benzene, m-xylene, o-xylene, and decane.
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